A gas turbine engine usually comprises a compressor, a combustion chamber and a turbine. The compressor delivers compressed air for use in the combustion chamber. In the combustion chamber a mixture of air and fuel is combusted by means of a burner in order to produce a hot gas stream which drives the turbine. Typically one or more burners are used. In this context it is important to monitor the flame to avoid instabilities of the combustion process. Therefore, it is desired to detect the presence of the flame and the intensity of the heat release rate from the flame. The heat release rate is an indication of the intensity of the chemical reaction and the stability of the flame.